MINI-RF BISTATIC OBSERVATIONS OF COPERNICAN CRATER EJECTA

The Mini-RF radar onboard the Lunar Reconnaissance Orbiter (LRO) is currently operating in a bistatic configuration using the Goldstone DSS-13 and Arecibo Observatory as transmitters in X-band (4.2-cm) and S-band (12.6 cm), respectively. A useful product for examining the scattering properties of the lunar surface that can be derived from backscattered microwave radiation is the Circular Polarization Ratio (CPR). The CPR of a surface can be interpreted as an indication of the roughness of the surface on the wavelength of the radar. Here, we examine the ejecta blankets of eight Copernican and one Eratosthenian aged crater on the lunar surface in both S- and X-band to examine the scattering properties of young crater ejecta. In particular, young craters that exhibit an opposition effect should present higher CPR at lower bistatic (phase) angles.

Of the Copernican craters examined, four (Byrgius A, Aristarchus, La Condamine S, Kepler) exhibit CPR characteristics suggestive of an opposition effect in S- and X-band, and one crater (Anaxagoras) exhibits an opposition surge in X-band but not S-band. The increase in CPR associated with this effect occurs near 2–4 degrees bistatic angle. These craters occur in both highlands and mare regions, and are all characterized as “young” from other observations. Three additional craters (Bouguer, Harpalus, Anaxagoras) exhibit CPR that remains relatively constant across bistatic angle in S-band. This may be for a couple reasons: 1) The craters are older (though most are still Copernican), and so the opposition effect will be less pronounced; or 2) insufficient data have been acquired to characterize the opposition behavior of the crater ejecta. An opposition effect may be present, and not yet observed. In S-band, Schomberger A, La Condamine S, and Kepler exhibit scattering properties as a function of bistatic angles that differ from the other observed craters, with areas of relatively constant CPR at various CPR values. We observed one Eratosthenian crater, Hercules, which shows no indication of an opposition response across phase angle space. Observing the scattering behavior of continuous ejecta blankets in multiple wavelengths may provide further information about the rate of breakdown of rocks of varying size to provide increased understanding of how impacts produce regolith on the Moon.